The .alpha.-naphthylpropionic acids are known from the literature for their biological properties; owing to the presence of the asymmetric carbon atom bonded to the naphthyl nucleus, they can exist both in the form of racemic mixtures and in the form of the corresponding d or l optically active isomers.
The d isomer of the compound of formula I in which R.sub.1 represents the methyl radical and R.sub.2 represents a hydrogen atom, namely the d-2-(6-methoxy-2-naphthyl)-propionic acid described in U.S. Pat. No. 3,904,682 and internationally known as naproxen (INN=International Nonproprietary Name), holds a noteworthy importance for its very good antiinflammatory properties.
Its preparation has been reported many times in the literature, mainly in the patent literature. Usually these methods contemplate the synthesis of d,l-2-(6-methoxy-2-naphthyl)-propionic acid, or a precursor thereof, and the subsequent resolution into the optical antipodes via formation of salts with optically active organic bases like cinchonidine, dehydroabietylamine, N-methyl-D-glucamine, N-alkyl-D-glucamines (see French Publication No. 2,035,846 and U.S. Pat. Nos. 3,683,015; 4,246,164; 4,246,193 and 4,423,244). All of these resolution methods possess more or less severe drawbacks. As an example, it is often necessary to carry out several recrystallizations for obtaining the salt of the desired isomer in the wanted purity degree; in addition, the purity degree of the mixture to be resolved remarkably influences the resolution itself.
The stereospecific synthesis of naproxen and, in general, of the optically active .alpha.-naphthyl-propionic acids (see European laid open application Nos. 81993 and 110671) has been tried for avoiding these drawbacks. To our experience, however, these procedures appear to involve a lot of problems, like the use of Grignard's reagents, the optical purity not always sufficiently high and the need to use optically active intermediates.
Therefore there is still the need of technically and economically valid resolution methods of the .alpha.-naphthylpropionic acids.